The present invention relates to inorganic molecular sieves, and more particularly to niobate-based octahedral molecular sieves (OMS) and methods for synthesizing such sieves and use thereof for contaminant removal.
Microporous inorganic materials have many industrial applications including ion, molecule, and gas separations; ion, molecule and gas sensing; and catalysis. Applications for microporous materials for metal ion separations include radionuclide separation from nuclear waste streams (e.g., 137Cs, 90Sr, 60Co), separation of transition metals from metallurgical wastes (e.g., Cr, Ni, Zn), and cleanup of heavy, toxic metals (e.g., Pb, Hg, Cd). See, e.g., U.S. Pat. No. 5,976,490 to Wendelbo, et al. (1999). Effective ion exchangers require properties such as high selectivity in variable chemical environments; radiation, chemical and thermal stability; and reusability through back-exchange.
In particular, the long-term storage, containment, and handling of radioactive strontium-90, cesium-137 and actinides in nuclear waste streams pose substantial engineering, scientific, and societal challenges. The wastes containing these radionuclides are multi-component and multi-phasic. Therefore, effective ion exchangers for removal of the radionuclides must exclude competing species, such as high concentrations of Na+ and other cations within the mixed wastes. Furthermore, appropriate ion exchangers must function in harsh chemical conditions and withstand extreme radiation environments created by absorbed radionuclides. Finally, the radionuclide-loaded ion exchanger should ideally be convertible to a stable, ceramic waste form.
Additionally, molecular sieves can be used to remove, isolate, and/or purify industrial contaminant metals, such as cobalt, nickel, and zinc.
In addition to the more common aluminosilicate sieves, many unique classes of tetrahedral framework, microporous materials useful for ion exchange have been synthesized including the phosphates, germanates, and arsenates. Additionally, microporous, octahedral framework structures such as molybdates, vanadates, and tungstates, and manganates have been shown to be useful ion exchangers. See, e.g., U.S. Pat. No. 5,518,707 to Bedard, et al. (1996); U.S. Pat. No. 5,681,973 to Hoelderich, et al. (1997); and U.S. Pat. No. 5,702,674 to Young, et al. (1997). Relatively few niobium- or tantalum-containing ion exchangers have been reported.
There still exist challenging ion selectivity problems including ion extraction from a highly acidic medium; separation of two ions which have very similar size or chemical behavior; back-exchange of ions, or reuse of ion exchangers; development of ion exchangers with extremely high ion exchange capacities for maximum efficiency; and prediction of ion selectivities of exchangers based on their structural properties.
The present invention comprises a niobate-based OMS having an octahedral framework wherein a portion of the niobium present in the framework is substituted by another octahedrally coordinated lower valence transition metal. A niobate-based OMS having an exchangeable alkali cation, such as sodium, can be synthesized by mixing a niobium alkoxide with a octahedrally coordinated lower valence transition metal alkoxide to form an alkoxide mixture; adding the alkoxide mixture to an alkali-containing solution to form a sol mixture; heating the sol mixture to form a reaction product in a parent solution; and separating the reaction product from the parent solution.
The niobate-based OMS having an exchangeable alkali cation can be used to remove a contaminant metal ion from a contaminated solution by exchanging the alkali cation for the contaminant metal ion. The contaminant metal ion-exchanged niobate-based OMS can then be backexchanged to concentrate the contaminant metal ion in an acidic solution. Alternatively, the contaminant metal ion can be immobilized by heating of the ion-exchanged niobate-based OMS to form a contaminant metal-loaded perovskite.